Fluorescence anisotropy decays determined for (dA)20.(dT)20 at 310, 330, 380 and 420 nm. Right panel: fluorescence anisotropy decays of dAMP (yellow) and TMP (green), which do not vary at the probed wavelengths.

The fluorescence of the DNA double stranded oligomer (dA)20.(dT)20 is studied at room temperature by fluorescence upconversion at times shorter than 10 ps. The profile of the upconversion spectra is similar to that of the steady-state fluorescence spectrum, showing that the major part of the photons is emitted within the probed time-scale. At all the probed wavelengths, the fluorescence decays are slower than those of the monomeric chromophores dAMP and TMP. The fluorescence anisotropy decays show strong wavelength dependence. These data allows us to conclude that energy transfer takes place in this double helix and that this process involves exciton states. The spectral and dynamical properties of the oligomer are compared to those of the polymer poly(dA).poly(dT), composed of about 2000 base pairs, reported previously. The oligomer absorption spectrum is characterized by a smaller hypsochromic shift and weaker hypochromism compared to the polymer. Moreover, the fluorescence decays of (dA)20.(dT)20 are a twice as fast as those of poly(dA).poly(dT) and its fluorescence anisotropy decays more slowly. These differences are the fingerprints of a larger delocalization of the excited states induced by an increase in the size of the duplex.